The present invention generally relates to improving the transmission of light through optical materials, such as spectacle lenses and, at the same time, reducing reflection of stray light that leads to glare from optical materials.
All uncoated optically transparent materials reflect a portion of incident light. The amount of reflection varies with the wavelength, polarization, and angle of incidence of the light as well as the wavelength-dependent refractive index of the transparent material. This Fresnel reflection is described by Maxwell's equations for electromagnetic radiation, as known to those practiced in the art of optics and described, for example, by M. Born and E. Wolf in Principles of Optics, New York, Pergammon Press (1980). It is also known that layers of transmissive materials with refractive indices different from that of the substrate can reduce the amount of reflection. The amount of this reduction depends on the wavelength-dependent refractive index of the coating materials and their thickness as well as the wavelength, polarization, and angle of incidence of the light. The design and manufacture of these coatings is thoroughly described in Chapters 3 and 9 of H. A. Macleod, Thin Film Optical Filters, (New York: McGraw-Hill) (1989).
The sensitivity of the human visual system also varies with the wavelength of light and its angle of incidence, as described, for example, in Color Science: Concepts and Methods, Quantitative Data and Formulae by Gunter Wyszecki and W. S. Stiles (New York:Wiley) (1982) and Visual Perception by Nicholas Wade and Michael Swanston (London: Routledge)(1991). It would be advantageous to exploit this human visual response function by designing and fabricating coated optical articles having coating thicknesses and compositions that result in a minimization of the perceived angular and wavelength variation of Fresnel reflection from the articles.
Prior methods for creating anti-reflection (AR) coatings employ physical vapor deposition in which high-energy electron beams are used to heat samples of inorganic materials such as titanium (Ti), silicon (Si), or magnesium fluoride (MgF.sub.2) in a vacuum chamber until they evaporate and deposit on the cooler substrate. The flux of evaporated material is isotropic and decreases with the square of the distance between the substrate to be coated and the evaporative source. The method requires a vacuum chamber whose dimensions are large compared to the dimensions of the substrate. Typical implementations of such methods are found in the Model 1100 High Vacuum Deposition System (Leybold-Hereaus GmbH, Hanau, Germany) and the BAK 760 High Vacuum Coating System (Balzers A. G., Liechtenstein). The rate of producing AR coatings with prior methods, as well as the high cost to purchase, operate, and maintain the apparatus, restricts their use to central production facilities. It is, therefore, desirable to provide a method for producing AR coatings on spectacle lenses that only requires compact, inexpensive hardware and can be performed at any location, such as a retail optician's office.
The evaporative method also causes heating of the substrate because convective cooling is inefficient in a vacuum and the hot elemental materials emit thermal radiation that may be absorbed by the substrate. The heating can cause substrate damage, such as internal stress and warping, especially with plastic substrates. It is, therefore, desirable to produce the AR coating at or near room temperature to avoid this damage.
Known AR coatings use one or more layers of refractory materials, such as inorganic oxides, nitrides, or fluorides, to achieve a reduction in reflection. Common thin-film materials used for such AR coatings are described in chapter 9 and Appendix I of Macleod, and include oxides of Al, Sb, Be, Bi, Ce, Hf, La, Mg, Nd, Pr, Sc, Si, Ta, Ti, Th, Y, and Zr. Macleod's tabulation also includes fluorides of Bi, Ca, Ce, Na, Pb, Li, Mg, Nd, a, and Th, as well as a few sulfides and selenides. A similar tabulation is found in table 4.1 on page 179 of Optics of Multilayer Systems (Sh. A. Furman and A. V. Tikhonravov, Editions Frontieres: Gif-sur-Yvette, France, 1992).
A problem with these AR coatings is that the mechanical characteristics of inorganic compounds, such as thermal expansion coefficient and elastic modulus, are very different from those of plastic substrates. It would therefore, be advantageous to produce an organic AR coating layer. It is also desirable to produce an AR coating layer whose properties are intermediate between known inorganic AR coatings and plastic substrates to act as a transition layer between organic and inorganic layers.
The reflectance of a coated optical article depends crucially on the thickness of the AR coating layer or layers. In the prior art, coating thickness has been monitored using a quartz microbalance in situ to measure the rate of mass deposition. The mass of the film does not enter directly into the equations that describe the optical properties of the layer. It would be highly advantageous to monitor film growth with an optical signal that is tied more directly to the AR properties of the coated article.